Broken bones, particularly severely broken bones, are often repaired using a technique known as internal fixation. The most commonly used fixation device is known as a bone plate or compression plate. This technique secures the bone fragments in place with fastening means such as screws placed through holes in the plate. The bone fragments may also be compressed together in an effort to facilitate bridging of the spaces between the bones.
Numerous different materials and designs have been used for the plates. It has been found that the initial and early strength of the plate is very important to immobilize the fragments during healing. Polymeric bone plates such as nonabsorbable polymer plates, e.g., K. Tayton et al., J. Bone and Joint Surg., 64-B(1), 105 (1982) and fiber reinforced nonabsorbable polymer composite plates, e.g., by G. B. McKenna, "The Development of Fiber Reinforced Polymer Composites for Orthopedic Applications," Ph.D. Thesis, University of Utah, 1976, have been described.
Absorbable polymer plates have also been described, e.g., by M. Virt, et al. in U.S. Pat. No. 4,279,249 and by Corcoran, et al., "The Development of a variable Stiffness, Absorbable Composite Bone Plate" in "Current Concepts of Internal Fixation of Fractures," H. K. Uhthoff, ed., Springer-Verlag, N.Y. (1980), where carbon fiber reinforced absorbable plates are described.
In most cases the material of choice for the bone plate remains a metallic alloy. Various steel alloys, generally varieties of stainless steel, are preferred. However, the use of steel plates for internal fixation has certain drawbacks. One of these is the phenomenon of stress-shielding, wherein stresses are exerted primarily on the plate rather the bone in the fracture region. This stress-shielding has been found to be the cause of significant bone resorption and consequent reduction of strength of the bone in the region of the healed fracture.
The use of polymeric bone plates has not satisfactorily solved the problem of stress-shielding, since the initial strength and rigidity of steel plates is desirable for most fractures. Attempts to solve the problem of stress shielding by providing polymeric washers or spacers in the bone fixation devices have recently been reported. Kummer and Coutts, U.S. Pat. No. 4,338,926 have described a bone fracture prosthesis wherein a bioabsorbable spacer or washer is used. These devices of the art are not successful in practice, presumably due to premature loss of structural integrity. They have been found to provide excessive callus formation, which indicates inadequate fixation and in some cases they permit catastrophic failure of the repair.